The present invention relates to high frequency telecommunications applications. Specifically, a voltage controlled oscillator (VCO) which may be used in a digital telephone for generating multiple bands of radio frequency signals and intermediate frequency signals is disclosed.
Voltage controlled oscillators are used for multiple purposes in the radio telecommunications industry. Specifically, in cellular telephone applications, voltage controlled oscillators are used to established a channel frequency within one or two bands according to the GSM digital telephone standard. The two frequency bands employed in the GSM standard are 850 to 900 MHZ, and 1800 to 1900 MHZ. In order to provide local oscillator signals, as well as transmit carriers, tunable voltage controlled oscillators are implemented in a frequency synthesizer application.
Local oscillator signals are usually generated with the same signal source used for generating a transmit frequency. In both applications, it is necessary to maintain the signal phase noise below a level which would otherwise increase the noise floor and limit the range of the cellular telephone. In one conventional voltage controlled oscillator design, a pair of bipolar transistors are utilized in a cross-coupled differential configuration having a current source connected to the emitter connections. A resonant collector circuit is digitally and analog tuned to control the oscillation frequency. Phase noise in the output signal is produced by the harmonic content of the signal produced by the current source which is re-modulated in the resonant collector circuit. In some of these applications, it is therefore desirable to suppress the harmonic content generated by the current sources.
In other applications of the voltage controlled oscillator, the presence of phase noise is not as critical. For instance, in quadrature phase modulation and demodulation systems, which use a lower intermediate frequency, phase noise is significantly less of a problem. The I and Q signals of a quadrature system are generated from a second harmonic signal using CML, or ECL logic, which is then divided by two, or by four, with a digital counter to produce symmetrical I and Q signals.
The present invention is directed to a voltage controlled oscillator which will perform in applications where phase noise is a critical design parameter or where phase noise is a lesser concern.